symfony2 prefill form field type collection

I have a 2 form types

In first form type I have a collection form type which uses another form type

in controller I can create e.g. 3 sets to the colection.

BUT!!!

How can I set default collection row data for combobox, when it comes from relational table (entity) and I want't to have for every collection row another set of data - based on sql and not the same where conditions.

thank you